1. Field of the Invention
This invention relates generally to a process for value-added processing of fats and oils to yield glycerol and glycerol derivatives. More particularly, the process converts glycerol to acetol and then acetol to propylene glycol (PG) to produce a propylene glycol with ultra-low amounts of ethylene glycol (EG). The process may yield glycerol-based products and glycerol derivatives, such as antifreeze and other products.
2. Description of the Related Art
Existing processes for the hydrogenation of glycerol to form other products are generally characterized by requirements for excessively high temperatures and pressures. For example, high temperatures may degrade the reaction products. Working pressures of several hundred bar create safety concerns and increase the capital costs of implementing these processes. Most of such processes yield substantial impurities that may necessitate costly purification steps to isolate the desired reaction products.
In one example, conventional processing of natural glycerol to propanediols uses a catalyst, for example, as reported in U.S. Pat. Nos. 5,616,817, 4,642,394, 5,214,219 and 5,276,181. These patents report the successful hydrogenation of glycerol to form propanediols. None of the processes shown by these patents provide a direct reaction product mixture that is suitable for use as antifreeze. None provide process conditions and reactions that suitably optimize the resultant reaction product mixture for direct use as antifreeze. None address the use of unrefined crude natural glycerol feed stock, and none of these processes are based on reactive distillation. Generally, existing processes
U.S. Pat. No. 5,616,817 issued to Schuster et al. describes the catalytic hydrogenation of glycerol to produce propylene glycol in high yield, such as a 92% yield, with associated formation of n-propanol and lower alcohols. Conversion of glycerol is substantially complete using a mixed catalyst of cobalt, copper, manganese, and molybdenum. Hydrogenation conditions include a pressure of from 100 to 700 bar and a temperature ranging from 180° C. to 270° C. Preferred process conditions include a pressure of from 200 to 325 bar and a temperature of from 200° C. to 250° C. This is because Schuster et al. determined that lower pressures lead to incomplete reactions, and the higher pressures increasingly form short chain alcohols. A crude glycerol feed may be used, such as is obtainable from the transesterification of fats and oils, but needs to be refined by short path distillation to remove contaminants, such as sulfuric acid that is commonly utilized in the transesterification process. The feed should contain glycerol in high purity with not more than 20% water by weight.
U.S. Pat. No. 4,642,394 issued to Che et al. describes a process for catalytic hydrogenation of glycerol using a catalyst that contains tungsten and a Group VIII metal. Process conditions include a pressure ranging from 100 psi to 15,000 psi and a temperature ranging from 75° C. to 250° C. Preferred process conditions include a temperature ranging from 100° C. to 200° C. and a pressure ranging from 200 to 10,000 psi. The reaction uses basic reaction conditions, such as may be provided by an amine or amide solvent, a metal hydroxide, a metal carbonate, or a quaternary ammonium compound. The concentration of solvent may be from 5 to 100 ml solvent per gram of glycerol. Carbon monoxide is used to stabilize and activate the catalyst. The working examples show that process yields may be altered by using different catalysts, for example, where the yield of propanediols may be adjusted from 0% to 36% based upon the reported weight of glycerol reagent.
U.S. Pat. No. 5,214,219 issued to Casale, et al. and U.S. Pat. No. 5,266,181 issued to Matsumura, et al. describe the catalytic hydrogenation of glycerol using a copper/zinc catalyst. Process conditions include a pressure ranging from 5 MPa to 20 MPa and a temperature greater than 200° C. Preferred process conditions include a pressure ranging from 10 to 15 MPa and a temperature ranging from 220° C. to 280° C. The concentration of glycerol may range from 20% to 60% by weight in water or alcohol, and this is preferably from 30% to 40% by weight. The reaction may be adjusted to produce significant amounts of hydrocarbon gas and/or lactic acid, such that gas generation is high when lactic acid formation is low and lactic acid formation is high when gas generation is low. This difference is a function of the amount of base, i.e., sodium hydroxide, which is added to the solvent. Alcohol reaction products may range from 0% to 13% of hydrocarbon products in the reaction mixture by molar percentages, and propanediols from 27% to 80%. Glycerol conversion efficiency exists within a range from 6% to 100%.